Plasma display panel including an opaque, reinforcing film

ABSTRACT

A plasma display panel of long life, more than several thousand hours, having a plurality of electrode pairs covered with dielectric layers facing to discharge gas space, and surface reinforcing film on one or both surface of the dielectric layers facing to discharge gas space. The surface reinforcing film is made from insulating materials, the chief component of which is an oxide of a rare-earth element.

United States Patent 1191 Osawa et al.

[ PLASMA DISPLAY PANEL INCLUDING AN OPAQUE, REINFORCING FILM [75] Inventors: Mitsuoki Osawa, Akashi; Toshinori I Urade, Kobe, bothof Japan [73] Assignee: Fujitsu Ltd., Japan 22 Filedz Dec. 20,-1972 [21] Appl. Noi: 316,981

[30] 7 Foreign Application Priority Data Dec. 29, 1971 Japan 46-75752 521 user 313/188;313/220; 313/221;

511 1m.c1. ..l-I0lj6l/35 [58] Field of Search .313/188, 201, 220, 221, .1 313/518 1 A References Cited UNITED STATES PATENTS 1 3,716,742 2/1973 Nakayama'ct a1. 313/188 X 1 7 'Sept 9,1975

3,787,106 1/1974 sch rmerho ....313/22oX OTHER PU BLfCATlONS Materials and Techniques for Electron Tubesfby Walter H. Kohl, Table 2.3, page, Reinhold Publishing Co., New York, N.Y. 1 960. r

Primary Examiner-Palmer C. Demeo Attorney, Agent, or FirmAllan Ratner; Nelson E. Kimmelman w 57 ABSTRACT j A plasma display panel of longlife, more than several i 1 Claim, 12 Drawing Figures a z z z u PATENTEUSEP 9 1975 saw 1 o g PATENTED EP 91975 904,906

H'g.3(15) Fig. 3(a) Fig. 5/0) 423 F QZU PATENTE'USEP 3, 904,906

sum 3 qg b 0 50 I00 260 400 hours PATENTEB SEP 9 I975 SHEET 1, 0?]? Fig. 6 (A) F /'g 6 (B) hours IO N PLASMA DISPLAY PANEL INCLUDING AN 1 I OPAQUE, REINFORCING; FILM.

are added to the sustaining signal. Wall charges correspending-to the polarity of the applied voltage are then formed. When the potential difference between the wall charges andthe following sustaining voltage be- .comes larger than the firing voltage, the firing spot is again produced, and the polarity of the wall voltage is reversed. Accordingly, once the firing is produced, a

sustaining "voltage lower .than the firing voltage can continuously produce thefiring spot. That is, the write information is memorized as the wall voltage and the display can be carried out. Such a gaseous discharge panel has becomeknown in the, art as a plasma panel,. and when utilized for display purposes is commonly referred to as plasma display panel.

Some structure and shape of said plasma display panel have been known in prior art. For instance, a pair of substrates each having a plurality of parallel linear conductors coveredwith dielectric means are posi tioned opposite each other so that said linear conductors of one-substrate are perpendicular to said linear conductors of the other substrate and a suitable gas is placed between the tyvo substrates. According to the --other example of prior art, a pairof electrodes whose shape relates to the shape of some alphanumeric letters are positioned opposite each other and suitable discharge gas. is filled between them.

. a The disadvantage of prior plasma display panel is short operation life.

.Two of the items which effect the life of plasma displaypanel are: (a).the material on the surface of the di- -electric layers has been dissolved and changed in quality, and; (b) the discharge gas includes some impurity or some components of the discharge gas have been absorbed into the surfaceof dielectric layers after a short .time 01 Operation. v

.- life of. plasmadisplay vpanel the most. Said ionbombardment removes the surface gas film, and dissolves and spatters the surface material. As a result, the value of the coefficient 'y( 'y,, y -ly,,,) of secondary electron emission from dielectric layers changes. Where y,,, 'y and 'y, are the coefficients of secondaryelectron. emission by ions,.light beams and semi-stable atoms, respectively. The change of the valuey causes ,a change in the value ofvconductivityofthe. surfacematerial of the dielectric layers. This result an the changing-ofthefiring voltage V and memory coefficient a;

further, the diameter of discharge spots changes and ,the transparency of the substrates is affected due to the blackening phenomenon of the surface of dielectric layers. These changes are larger for the discharge cells which have been fired more times. Accordingly a dif-., ference inthe frequency of use of discharge cells in one plasma display panel destroys the uniformity of electric characteristics of the discharge cells, and this lack of uniformity limits the even life of the plasma display panel.

In above explanation, the value a is defined as:

where V, is the firing voltage, and V is the minimum limit of sustaining voltage. Further the value 04,, is defined as:

where 8V, is the difference between maximum and minimum firing voltages of all cells on the whole panel, and 5V is the difference between maximum and minimum sustaining voltagesof all cells on the whole panel. The value a is preferably as small as possible.

Generally speaking, the material for dielectric layers is low melting point glass which includes much lead that is to say, said glass includes 60 of oxidelead (PbO). Said glass is superior in such electrical characteristics as firing voltage V and memory coefficient a, and is superior with respect to easy baking to a substrate and easy manufacturing of containers. Further said glass is superior with respect to small differences of expansion coefficients between glass and substrate, wettability, viscosity transparency and softening temperature. However, the disadvantage of said glass for dielectric layers of a plasma display panel is that the connection between lead and the oxygen of oxidelead is not strong. Therefore, ion-bombardment due to discharge destroys said connection between lead and oxygen, which changes the quality of the surface and releases oxygen atoms into the rare discharge gas. The above is the reason why the life of plasma display panels of prior art is short.

The object of the present invention is to provide an improved plasma display panel which overcomes the above-mentioned drawbacks.

Another object of the, present invention is to provide an improved plasma display panel of long life.

A further object of the present invention is tqprovide an improved plasma display panel whose V a and an do not change over a period of long operation.

A plasma display panel according to the present invention to accomplish each of the above objects is provided with surface reinforcing or protection layer on at least one surface of the dielectric layers facing the discharge gas, Said surface protection layers are dielectric materials in which the main-component is made from oxideof rare-earth elements.

Further features and advantages of the present invention will be apparent from the ensuing description with reference to the accompanying drawings; to which, however, the scope of the invention-is in no way limited. 1

FIG. 1 is a perspective view illustrating the construction of a typical display panel utilizing a gas discharge;

FIG. 2 is a sectional view illustrating the construction of a typical displaypanel utilizing a gas discharge shown in FIG. 1;

FIGS. 3(A) through (D) show another embodiment of a typical display panel utilizing a gas discharge;

FIGS. 4(A) and (B) show life characteristic curves of prior plasma display panels;

FIG. 5 shows one embodiment of the present invention in an enlarged sectional view illustrating the construction of a typical display panel utilizing a gas dis' charge according to the present invention;

FIG. 6(A) and FIG. 6(B) show life characteristic curves of a plasma display panel according to the present invention;

FIG. 7 shows another embodiment of the present invention in an enlarged sectional view illustrating the construction of plasma display panel utilizing a gas discharge according to the present invention.

Referring to FIG. 1, a display device 1 utilizing a gaseous discharge has a pair of supporting substrates of electrodes 2 and 2a. The supporting substrate of electrodes 2 provides a group of electrodes 3 arranged in columns, which is parallel to a vertical axis; dielectric layer 4 covers the group of electrodes 3. The supporting substrate of electrodes 2a provides a group of electrodes 30 arranged in rows, which is parallel to a horizontal axis; dielectric layer 411 covers the group of elec trodes 3a. The supporting substrates of electrodes 2 and 2a are positioned in spaced parallel relation to the groups of electrodes 3 and 3a respectively. As shown in FIG. 2 these parallel rows and columns are separated from each other by a gap 5. This gap 5 is filled with a rare gas having suitable pressure and capable of ionization. When the device 1 is utilized for display purpose, it is necessary that, at least, one of the substrates 2, 2a and one of the dielectric layers 4, 4a are transparent.

In the above-mentioned display device utilizing gas discharge 1 shown in FIGS. 1 and 2, when an electric voltage higher than the firing voltage V; is selectively applied between the groups of electrodes in columns 3 and rows 311, each cross point of the electrodes in columns and row discharges into the gap 5 filled with an ionizable gas. At the time of this discharge, a wall charge is formed near the above-mentioned cross point in the dielectric layers 4 and 4a. With the effect of this wall charge, the discharge which is once generated is sustained with the pulsive sustaining voltage V, smaller than the firing voltage V and is continued. That is, the information which inputs as the voltage exceeding the firing voltage V is kept in memory by the abovementioned wall charge.

FIG. 3(A) shows the other embodiment of plasma display panel utilizing gas discharge of prior art. In FIG. 3(A), common electrode 6 and segment electrodes 7a 7g divided into seven sections are positioned face to face and rare gas is filled between common electrode 6 and segment electrodes 7a 7g. The surface of electrodes 6 and 7a 7g are covered with dielectric layers (not shown) like the embodiment of FIG. 1 and FIG. 2. The display device which has the structure of FIG. 3(A) can be used to show numerical letters. That is to say, selective discharge between common electrode 6 6 and segment electrodes 7a 7g provides the display of numerical letters like FIGS. 3(B) (D), where FIG. 3(B) shows 4", FIG. 3(C) shows 6", and FIG. 3(D) shows 8.'Fo'r instance, in FIG. 3(B), numerical letter 4" is displayed by discharge between common electrode 6 and segment electrodes 7b, 7c 7d and 7f.

A structure of the plasma display panel other than that shown in FIG. I FIG. 3 is also possible. For in stance a single plane discharge type or lateral discharge type plasma display panel which has a plurality of pairs of electrodes placed on the same side of the discharge gas is possible. (The plasma display panel of FIG. 1 and FIG. 2 is called cross point discharge type or vertical discharge type as plurality of electrodes are placed on both sides of discharge gas). Further, a self-shift type plasma display panel is possible which has a so-called chain reaction effect or fusing effect caused by ionized gas near discharged spots. In the self-shift type, firing voltage of spots near the discharged spot is lower than that of others because of presence of ionized gas. By utilizing said effect positively, in a self-shift plasma display panel, discharged spots move automatically towardsa predetermined direction if a polyphase voltage is applied to the electrode.

FIGS. 4(A) and (B) show characteristic curves of firing voltage V;, memory coefficient a, value 04,, against operation time of prior plasma display panels. In FIG. 4(A), curves (1 and b show the characteristics V, and a of the spots discharged continuously, while curves a and b show the characteristics V and a of the undischarged spots. It should be understood from FIG. 4(A) and FIG. 4(B) that is prior plasma display panels the values of V,, and 01,, for operation spots start to change after about 50 hours. Said change causes a shortening of the life of the plasma display panel itself.

The reason the life of a prior plasma display panel is as short as shown in FIG. 4(A) and FIG. 4(B), is the change in the quality of the dielectric material on the surface of dielectric layers facing the discharge gas as explained above.

In general, the condition required in the material of the dielectric layers facing the discharge gas of a plasma display panel is as follows.

a. The coefficient 'y of the second electron emission should be large in view of the lower firing voltage VJ- b. The surface resistance and volume resistance should be large enough to hold a wall charge for a long enough period of time.

c. The equivalent capacitance of the dielectric layers should be small enough to restrict the value of discharge current, that is to say, the dielectric layers should have the property that it is possible to manufacture thick layers in order to obtain small capacitance. d. The material should have enough dielectric strength. e. The material should have enough strength against ion-bombardment.

f. Wettability and the difference of expansion coefficient between substrates and dielectric layers should be reasonable, and further chemical property, softening I temperature and transparency should be reasonable.

Among the above conditions, (a) and (e) relate to the property of the surface of dielectric layers only, and these two conditions have a large effect on the life of the plasma di spla'y panel. For conditions (b), (c), (d) and (f) dielectric material which has been successfully employed many times previously can be preferably used.

Accordingly, the plasma display panel of the present invention has the structure that at least one surface of dielectric layers facing the discharge gas is covered with thin surface reinforcing film having properties which satisfy said conditions (a) and (e).

The desired properties of a surfacereinforcing film are strong connection with oxygen, small sputtering coefficient, good insulation, transparency and easy manufacture. One material for surface reinforce film which satisfies said desired properties is oxide-cerium (CeO which has strong connection with oxygen, strong oxidation property and small sputtering coefficient. Oxidecerium is also a good dielectric material and is chemically stable.

1000 5000 A thick of oxide-cerium is attached to the surface of dielectric layers of about 1. thick by evaporation technique heated by resistance heating.

FIG. 5 shows one embodiment of the present invention in an enlarged sectional view of a plasma display panel having a surface reinforcing film according to the present invention. In FIG. 5, 9 is a first substrate on which a plurality of parallel spaced conductors are positioned as a first group of electrodes 10. Said group of electrodes 10 are covered with a first dielectric layer.

11, whose surface facing the discharge gas space 13 is further covered with a first surface reinforcing film 12. 17 is a second substrate on which a plurality of parallel spaced conductors are positioned as a second group of electrodes 16. This second group of electrodes 16 are covered with a second dielectric layer 15, whose surface facing the discharge gas space 13 is further covered with a second surface reinforcing film 14. Two substrates 9 and 17 are placed so as to face each other across the discharge gas space 13 so that the two groups of electrodes 10 and 16 are perpendicular to each other.

FIGS. 6(A) and (B) show characteristic curves of firing voltage V;, memory coefficient a and value a versus operation time of the plasma display panel of FIG. 5. ln FIGS. 6(A) and (B) time axis (lateral axis) is marked with logarithm degree. In FIG. 6(A), curves at and b show the characteristics V, and a of the spots discharged continuously, while curves a and b show characteristics V, and a of the undischarged spots.

It should be understood from FIG. 6(A) and FIG. 6( B) that the value of V oz and a, of the plasma display panel according to the present invention hardly change for a long time for discharged and undischarged spots. Therefore, life of the plasma display panel is considerably improved by the present invention, said life being more than several thousand hours.

In addition to long life, some of the advantages of the plasma display panel having surface reinforcing film of oxide-cerium according to the present invention are: easy evaporation property; firing voltage V, memory coefficient a and property of discharged spots being about equal to prior display panels; desirable transparency, and; easy treatment. Other than oxide-cerium said surface reinforcing film can be made from some oxides of rare-earth elements such as Yttrium (Y), Europium (Eu), Gadolinium (Gd), Teribium (Tb) or Dysprosium (Dy); Holmium (Ho), Erbium (Er), Thulium (Tm), Ytterbium (Yb) or Lanthanium (La); Scandium (Sc) or Thorium (Th). For the method of attaching said surface reinforcing film on the surface of dielectric layers it is possible to use, for instance, some evaporation technique like resistance heating evaporation, electron beam heating evaporation, high-frequency sputteririg evaporation, melt and spray method, sedimentation method or spray method. In said method of attaching the film on the surface of the dielectric layers is also possible to obtain oxide by an anode-oxidize method after attachment of the metal element to said surface, or to obtain oxide film by activated sputtering of metal in a rare gas atmosphere including oxygen. Further, the evaporation of mixtures like Y O TiO and thin film made from mixture Dy O B 0 SiO of the weight ratio 10 1 10, are possible. If oxidelanthanium (La O is used for the surface reinforcing film, the operation should be done in dry atmosphere as it is hygroscopic.

FIG. 7 shows another embodiment of the present invention in an enlarged sectional view of a plasma display panel according to the present invention. One

v point in which the embodiment of FIG. 7 differs from that of FIG. 5 is that the second surface reinforcing film 14 is removed and only the first surface reinforcing film 12 is provided in the embodiment of FIG. 7. Though the plasma display panel of FIG. 7 has surface reinforcing film on only one dielectric layer, the life property of it is almost the same as that of FIG. 5.

The reason the plasma display panel of FIG. 7, which has surface reinforcing film on only one dielectric layer, can obtain almost the same long life as that of FIG. 5, which has surface reinforcing films on both dielectric layers, is as follows.

The surface reinforcing film provided on one dielectric layer protects not only the layer which has reinforcing film, but also the layer which does not have reinforcing film, because said reinforcing film is sputtered by electron and/or ion during discharge. Consequently, the sputtered materials cover and protect little by little the dielectric layer which has no surface reinforcing film, and thus something like sputtered protection film is provided on the dielectric layer. Said sputtered protection film may be composed of a mixture of the material of the protection film and that of the prior dielectric layers, as the prior dielectric layer may also be sputtered. Further, the sputtered protection film may also cover the protected surface.

The other advantage of the embodiment of FIG. 7 is that an opaque film on the one dielectric layer may be used as a reinforcing film. In addition the manufacturing cost of the plasma display panel of FIG. 7 may be lower than that of FIG. 5, as only one surface reinforc ing film is needed.

Further, in the case where a change in quality of discharge gas disturbs the characteristics of the plasma display panel due to the decomposition of oxygen gas from oxide by the sputtering and/or ion attack from the surface reinforcing film and/or dielectric layer in FIG, 5 or FIG..7, getter which observes oxygen gas gently may provide longer life of the plasma display panel.

From the foregoing it will now be apparent that an improved plasma display panel with long life has been provided. It should be understood of course that the eembodiments disclosed here are merely illustrative and are not intended to limit the scope of the invention. Some modification within the spirit of the invention may be possible. For instance, the whole dielectric layer may be made of the same material as that of the surface reinforcing film, instead of said layer being covered with reinforcing film. Further, the surface reinforcing film may be applicable not only to cross point discharge type or vertical discharge type plasma display panels but also to all kinds of plasma display panels including single plane discharge type or lateral discharge type, and that of FIG. 3.

What we claim is:

1. A plasma display panel comprising;

a. a first and a second dielectric substrate, said first and second substrates being spaced from each other and sealed, a discharge gas contained within said sealed substrates forming a discharge gas space, at least one of said substrates being transpar- 

1. A PLASMA DISPLAY PANEL COMPRISING, A. A FIRST AND A SECOND DIELECTRIC SUBSTRATE SAID FIRST AND SECOND SUBSTRATES BEING SPACED FROM EACH OTHER AND SEALED, A DISCHARGE GAS CONTAINED WITHIN SAID SEALED SUBSTRATES FORMING A DISCHARGE GAS SPACE, AT LEAST ONE OF SAID SUBSTRATES BEING TRANSPARENT, B. FIRST AND SECOND ELECTRODES MOUNTED ON SAID FIRST AND SECOND SUBSTRATES, RESPECTIVVVELY, 